As a rule, the greater the number with the x the thinner the diameter. The pound test varies from brand to brand. Once you get above 0x, you would have sizes larger, x1 or bigger When the X comes first than the larger number with it the heavier it is, but typically the leader or tippet is just reffered to by its pound test.

There is a good guideline for understanding the "X" designation in fly tippets... the rule of 11. These ratings were probably developed in the days of twisted gut leaders but they are useful in that you can find the leader you want quickly regardless of brand or pound test for purposes of matching your leader and fly with a tippet.

Rule of 11...

0X == .011

Subtract .001 for each increment, thus 1X = .010

2X = .009
3X = .008, etc... see chart below

Note that the X comes to the front when you exceed the 0X standard, in other words put the X in front if the size increases from 0X.

I've usually seen the sizes larger than 0X referred to as 01X, 02X, 03x, etc. So you might see this nomenclature instead of that presented by Juro above.

My understanding is that the "X" designation system is a carry over from the old days when tippet was made of gut and drawn through a die to creat the desired diameter. The" X" referred to the number of times the gut was drawn through a die. So, a 3X was drawn through three times, a 4X four times, etc. Makes sense if you think of the X like a multiplication symbol, e.g., 2 x 2 = 4 = 2 times 2.